1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device. More particularly, the present invention relates to a method of fabricating a flash memory.
2. Description of the Related Art
Conventional non-volatile flash memory devices such as erasable programmable read only memories (EPROM), electronically erasable programmable read only memories (EEPROM), and flash memories do not lose their stored information when power is lost. Because these devices have superior information storage capacity, they are the subjects of ongoing related research.
Electronically erasable programmable read only memory (EEPROM) is a kind of non-volatile memory. Most EEPROM cells have two gates, including a floating gate made of polysilicon to store charge and a control gate used to control information storage. The floating gate is usually in a "floating" state. That is to say, no lines are connected to it. The control gate is often connected to a word line. A tunnel oxide layer and a dielectric layer are arranged between the substrate and floating gate and the floating gate and control gate, respectively. Additionally, source and drain regions are arranged in the substrate on both sides of the control gate.
Generally, as the gate-coupling ratio between the floating gate and the control gate increases, the work voltage necessary to operate the memory transistor decreases. As a consequence, the operational speed and efficiency of the flash memory increase tremendously. Methods for increasing the gate-coupling ratio (GCR) include: increasing the overlapped area between the floating and the control gate, reducing the thickness of the dielectric layer between the floating gate and the control gate, and increasing the dielectric constant (k) of the dielectric layer between the floating gate and the control gate. Generally, to achieve an increase in the overlapped area between the floating and control gates and thus increase the gate-coupling ratio (GCR), the size of the floating gate has to be increased. However, this is not desirable for the demands of today's highly-integrated technologies.